Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles.
Coating systems capable of satisfying the above requirements typically include a metallic bond coat that adheres the thermal-insulating ceramic layer to the component. Metal oxides, such as zirconia (ZrO.sub.2) partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides, have been widely employed as the material for the thermal-insulating ceramic layer. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure. Bond coats are typically formed of an oxidation-resistant diffusion coating such as a diffusion aluminide or platinum aluminide, or an oxidation-resistant alloy such as MCrAlY (where M is iron, cobalt and/or nickel). Aluminide coatings are distinguished from MCrAlY coatings, in that the former are primarily aluminide intermetallic while the latter is a metallic solid solution that contains a mixture of phases, including NiAl.
Though significant advances have been made with coating materials and processes for producing both the environmentally-resistant bond coat and the thermal-insulating ceramic layer, there is the inevitable requirement to remove and replace the ceramic layer under certain circumstances. For example, removal may be necessitated by erosion or impact damage to the ceramic layer during engine operation, or by a requirement to repair certain features such as the tip length of a turbine blade. Removal of the ceramic layer may also be necessitated during component manufacturing to address such problems as defects in the coating, handling damage and the need to repeat noncoating-related manufacturing operations which require removal of the ceramic, e.g., electrical-discharge machining (EDM) operations.
The current state-of-the-art repair methods often result in removal of the entire TBC system, i.e., both the ceramic layer and bond coat, after which the bond coat and ceramic layer must be redeposited. Prior art abrasive techniques for removing thermal barrier coatings have generally involved grit blasting, vapor honing and glass bead peening, each of which is a slow, labor-intensive process that erodes the ceramic layer and bond coat, as well as the substrate surface beneath the coating. With repetitive use, these removal processes eventually destroy the component by reducing the wall thickness of the component. Damage is particularly likely when treating an air-cooled turbine blade, whose surface includes cooling holes from which cooling air is discharged in order to cool the external surfaces of the blade.
Consequently, significant effort has been directed to developing nonabrasive processes for removing ceramic coatings. One such method is an autoclaving process in which the ceramic coating is subjected to elevated temperatures and pressures in the presence of a caustic compound. This process has been found to sufficiently weaken the chemical bond between the ceramic and bond coat oxide layers to permit removal of the ceramic layer while leaving the bond layer intact. However, this process is incapable of removing ceramic from the cooling holes of an air-cooled turbine blade. Another method for removing a ceramic layer involves the use of a high pressure waterjet, as reported in U.S. Pat. No. 5,167,721. While this waterjet technique is described as not removing the bond coat, in practice the waterjet can inflict significant damage to bond coats and particularly diffusion aluminide bond coats, which are brittle beneath about 1200.degree. F. (about 650.degree. C.). Damage generally occurs by the fracturing of brittle phases in the bond coat, such as PtAl.sub.2 phases of a platinum-aluminide bond coat, and/or the additive layer, which is the outermost bond coat layer containing an environmentally-resistant intermetallic phase MAl, where M is iron, nickel or cobalt, depending on the substrate material. Similar to grit blasting techniques, bond coat damage from the waterjet process is particularly likely when treating an air-cooled turbine blade. Damage can be acute around the cooling holes of these blades because ceramic within the holes is anchored by compressive stresses that develop when the newly coated component cools from typical coating temperatures for ceramic deposited by PVD techniques. Consequently, to remove the ceramic from a cooling hole, excessive dwell times are required to overcome this strong mechanical bond as well as the chemical bond between the ceramic and oxide layers, resulting in significant damage or removal of the bond coat in and around the cooling holes.
Accordingly, what is needed is a process capable of removing a ceramic layer from a cooling hole of an air-cooled component without damaging the cooling hole.